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O: Oberflächenphysik
O 27: Elektronische Struktur (IV)
O 27.4: Talk
Thursday, March 30, 2000, 12:00–12:15, H37
HREELS study of electronic excitations in PTCDA monolayers on Ag(110) and Ag(111) — •V. Shklover1, F.S. Tautz2, S. Sloboshanin2, L. Kilian1, M. Sokolowski1, J.A. Schaefer2, and E. Umbach1 — 1Experimentelle Physik II, Universität Würzburg, Am Hubland, 97074 Würzburg — 2Institut für Physik, Technische Universität Ilmenau, PF 100565, 98684 Ilmenau
By HREELS we have investigated electronic No-dqexcitationsNo-dqin UHV vapour-deposited films of the organic semiconductor molecule PTCDA on Ag(110) and Ag(111). The monolayer spectra differ drastically from the corresponding multilayer spectra. Firstly, the excitonic structure of PTCDA around 2.5 eV observed for multilayers is not present. Secondly, new features emerge, for Ag(110) at 0.79 eV and for Ag(111) at 0.36 eV; moreover, the intensity of the feature at 0.36 eV on Ag(111) is more than 5 times larger than that at 0.79 eV on Ag(110). We interpret these structures as transitions between rehybridized HOMO and LUMO orbitals which arise because of the chemisorptive bond with the substrate being different for both substrates. The fact that the adsorbate induced HOMO level on Ag(111) lies at the Fermi level explains the enhanced intensity as a resonant coupling between an electronic excitation and the C-H stretch vibrational mode of PTCDA. Interestingly, by deposition at 100 K on Ag(111) the feature is not enhanced but shifted to 0.7 eV. It appears again by annealing of the film up to room temperature. We explain this behavior as formation of a metastable precursor state with a different chemisorptive bond and a slighly different adsorption site, as observed by SPA-LEED.